Lightwave networks are increasingly being used to transfer information around the world. Lightwave networks include a number of stations, or nodes, that are interconnected by stretches of waveguides, typically optical fibers. Inside an optical fiber, pulses of light are transferred over long distances. In order to maintain the power of optical signals as they propagate through long stretches of optical fiber and/or other hardware devices, optical amplifiers are added in various locations in an optical network to maintain the energy level required for reliable optical communication.
In optical communications systems, a common optical amplifier is an erbium-doped fiber amplifier (EDFA). An EDFA is a segment of optical fiber integrated into the optical path to transfer optical energy to a passing signal when the EDFA is contacted by a pump source, which is typically a 980 nanometer or a 1480 nanometer laser pump source. The laser pump excites the EDFA, which in turn transfers optical energy to the passing signal, creating an amplified signal or, at a minimum, passing the sign attenuation. When pump energy is not applied to an EDFA, the EDFA may attenuate a passing optical signal by as much as 50 dB. The term pump energy used in conjunction with an EDFA or erbium-doped fiber (EDF) refers to the power associated with the pump wavelength of the EDFA or EDF.
Optical switches have been developed that incorporate the amplifying characteristics of EDFAs. FIG. 1 illustrates an optical switch utilizing an EDFA, as disclosed in U.S. Pat. No. 5,655,036, entitled "Branching Unit for Telecommunications Optical Cable Systems," issued to Webb. An optical attenuator 10 is combined with an EDFA 12 along an optical path to function as a switch to selectively prevent optical signals from propagating from an input fiber 14 to an output fiber 16. In order to allow optical signals to pass from the input fiber to the output fiber of the switch, the EDFA is powered by a laser pump 18. That is, the optical switch is controlled by turning the laser pump on and off, as needed. The laser pump is electronically turned on and/or off by a pump laser on/off circuit 20.
A disadvantage of the Webb system is the requirement of direct control of the pump laser output power to effectuate a switching event in a single switch. In addition, an optical switch requiring pump output power control has limited scalability. Both limitations may cause problems when rapid on/off switching is needed.
In addition to optical amplifiers, other optical devices such as wavelength stabilized lasers, wavelength division multiplexers, demultiplexers, and gain equalizers are used in lightwave communications systems. In many of the devices, fiber Bragg gratings (FBG) are used to reflect narrow bands of light energy in order to achieve a desired result. An FBG reliably reflects light energy by means of periodic changes or perturbations along the core of an optical waveguide. The perturbations create changes in the refractive index of the core of the waveguide. At each change, a portion of the optical energy is reflected, inducing interference in a constructive manner. The degree of change in refractive index along the core of an optical fiber, the grating period, and length of the FBG are factors that determine the range of wavelengths that will be reflected, as well as the efficiency of reflection.
FBGs are effective devices for filtering narrow wavelength bands of light energy in optical fibers. In addition, an FBG can be easily adjusted or tuned, in place, to cause the FBG to reflect a different wavelength. Tuning of an FBG's wavelength band of reflectivity typically involves thermally or mechanically changing the periodicity of the periodic perturbations of the FBG.
In contrast to FBGs, bandpass filters such as Fabry-Perot filters are used in optical fibers to reliably reflect a wide wavelength band of optical energy, while allowing a narrow wavelength band of optical energy to pass. Bandpass filters are also adjustable with respect to the passing wavelength band.
As lightwave network technology continues to develop, more advanced optical switches are needed to create new and improved optical devices. The narrow wavelength band filtering properties of FBGs may be utilized to create the advanced optical switches. Although the switch disclosed by Webb works well for its intended purpose, in order to create new and improved optical devices, what is needed is an optical switch that does not require direct control over pump laser output power. In addition what is needed is an optical switch that can be scaled up to create useful optical devices.